This invention relates to thermal mass transfer printing, and, in particular, to a novel receptor sheet for such printing.
In thermal mass transfer printing, an image is formed on a receptor sheet by selectively transferring image-forming material thereto from a donor sheet. Material to be transferred from the donor sheet is selected by a thermal printhead, which consists of small, electrically heated elements which are operated by signals from a computer in order to transfer image-forming material from the donor sheet to areas of the receptor sheet in an image-wise manner.
There are essentially two broad classes of donor sheet-receptor sheet systems--(1) chemical reaction systems and (2) mass transfer systems.
In chemical reaction systems, the image is formed upon the receptor sheet as a result of the imagewise transfer of some chemical reactant from the donor sheet. An example is the transfer of a mobile molecule, such as a phenol, to the receptor sheet, which bears a leuco compound thereon. The phenol is transferred by being volatilized by the heat from the thermal printhead, and, upon reaching the receptor sheet, reacts with the leuco compound to convert it from the colorless to the colored form. Alternatively, the phenol can be on the receptor sheet and the leuco compound can be on the donor sheet.
In mass transfer systems, no color-forming chemical reaction takes place. Instead, the image is formed simply by the transfer of the coloring material itself.
In U.S. Pat. No. 3,898,086, a wax composition is transferred imagewise to a receptor film by means of heat which melts the wax and allows it to readhere, upon cooling, to the receptor film. The final step in this process is the separation of the donor sheet and receptor film by pulling them apart. The donor sheet, which bears a negative image, is then used as a visual transparency. The receptor film used in this process is not of sufficient transparency to be useful for projection. In another wax transfer process described in DE No. 3,143,320, pressure, rather than heat, is used to effect the transfer. Such pressure can come from a pen, pencil, or typewriter, or other pressure-applying device. This system is not adaptable to thermal printing processes with the type of apparatus currently in use.
A typical donor sheet that is useful with thermal printers currently on the market comprises a paper or film backing having a layer of a pigmented wax coated thereon. Such a sheet is described in Seto, et al., U.K. Patent Application No. GB 2,069,160 A. The layer of transfer material comprises 1 to 20% by weight coloring agent, 20 to 80% by weight binder, and 3 to 25% by weight softening agent. A solid wax having a penetration of 10 to 30 is preferred as the binder. The softening agent is an easily meltable material such as polyvinyl acetate, polystyrene, etc. In order for image transfer to occur in such a system, the wax must soften sufficiently so that it can be released from its backing, and transfer to the receptor sheet in imagewise manner, but it should not become so soft as to run or move about on the receptor sheet. At the instant of transfer, the pigmented wax is held between the competing forces of the backing of the donor sheet and the image receptive surface of the receptor sheet. If the receptor sheet is paper, the transfer occurs by a combination of adhesion, capillary action, and mechanical intermingling of wax and paper fibers. Because the porosity of paper makes the adhesion area of the paper receptor sheet much greater than the surface area occupied by the image on the donor sheet, release from the backing of the donor sheet and transfer to and adhesion on the paper receptor sheet is favored.
If the receptor sheet is polymeric film, transfer depends entirely upon the adhesion of the softened pigmented wax to the relatively smooth film surface. In the absence of the mechanical coupling of pigmented wax to the receptor sheet, such as is provided by the pores of a paper surface, the adhesive properties of the polymeric film surface become critical. Adequate imaging will occur only if the adhesion between the pigmented wax and the film surface of the receptor sheet overcomes the adhesion of the wax to the backing of the donor sheet. It has been found that pigmented wax from a donor sheet does not reliably adhere to bare, untreated polyethylene terephthalate film because lack of compliance of the surfaces of the donor sheet and receptor sheet makes contact between pigmented wax of the donor sheet and image receptive surface of the receptor sheet difficult. Corona treatment of the polyethylene terephthalate film just prior to imaging improves wax transfer, but this is not a practical alternative for use in an office setting. A further difficulty in the use of bare, untreated polyethylene terephthalate film for thermal transfer imaging is the heat capacity of this material, which limits the range of useable calipers to a maximum of approximately 2 mils (50.8 micrometers). Films having calipers greater than this cannot be heated sufficiently to achieve the temperature needed for imaging.
Ideally, a receptor sheet made of polymeric film should have the characteristics of high clarity, reliable feedability in conventional thermal mass transfer printers, good handleability, and good adhesion of image-forming material. Haze should be below 15% as measured on the Gardner hazemeter, a level of 10% or less being preferred. The receptor sheet should preferably add no detectable color to the printed image. The receptor sheet should preferably feed reliably through the printer without sticking or jamming and without the need for any modification to printers originally designed to make paper copies. The receptor sheet should preferably be capable of being easily handled, without stickiness or susceptibility to excessive fingerprinting, which would add visible defects to the sheet noticeable upon projection. This is particularly important with respect to transparencies made from the receptor sheet. Transfer of pigmented wax from the donor sheet to the receptor sheet should preferably be complete in the areas to be imaged, and there should not be excessive wax transfer in areas to be free of the printed image. Sensitivity to small dots and thin lines is a desired feature and solid dark areas should appear solid when projected. The receptor sheet should also provide acceptable images for any caliper of film in the range of 1.5 to 7.0 mils.